1. Field of the Invention
The present invention relates generally to local oscillators, and more particularly, to a dielectric oscillator for use in an antenna unit (Low Noise Block downconverter, hereinafter referred to as "LNB") for receiving signals in the Ku band or higher frequency bands (such as Ka band).
2. Description of the Background Art
The Ku band about in the range of receiving frequencies from 10 GHz to 13 GHz has been used for satellite broadcasting and satellite communication. A typical Ku band satellite broadcasting receiving system will be now described in conjunction with accompanying drawings, where the same reference characters represent the same or corresponding portions.
Referring to FIG. 11, the Ku band satellite broadcasting receiving system is divided into an outdoor part 200 and an indoor part 210. Outdoor part 200 includes an antenna 201 and an LNB 202 connected thereto, while indoor part 210 includes an indoor receiver 204 and a television set 209. LNB 202 amplifies an electric wave received from antenna 201 with reduced noise and supplies a signal at a sufficient level with a low noise to indoor receiver 204 connected through a coaxial cable 203. Indoor receiver 204 includes a DBS tuner 205, an FM demodulator 206, a video and audio circuit 207 and an RF modulator 208. A signal applied to indoor receiver 204 through coaxial cable 203 from LNB 202 is processed by these circuits and applied to television set 209.
As a typical Ku band receiver LNB, a domestic CS receiving LNB will be now described. Referring to FIG. 12, an incoming signal at an input frequency in the range from 12.2 GHz to 12.75 GHz is received at an antenna probe 251 in a waveguide, amplified by a low noise amplifying circuit herein after simply as "LNA") 252 with reduced noise, and then passed through a band pass filter (hereinafter simply as "BPF") 253. BPF 253 allows a desired frequency band to pass therethrough in order to remove a signal in an image frequency band.
A signal passed through BPF 253 is input to a mixing circuit (hereinafter MIX 254 together with an oscillation signal of 11.2 GHz from a local oscillator (LO) 256, and frequency-converted into a signal in an intermediate frequency band from 1000 to 1550 MHz at MIX 254. The resultant signal is amplified by an intermediate frequency amplifying circuit (hereinafter as "IF AMP") 257 to have appropriate noise and gain characteristics and output from an output terminal 261. A power supply 258 is a power supply to provide electric power to LNA 252, IF AMP 257 and local oscillator 256.
In the Ku band satellite broadcasting receiving system as described above, local oscillator 256 used in LNB 202 is a critical part which determines the performance of LNB 202. A dielectric resonator oscillator (DRO) generally called a drain ground band reflective type dielectric oscillator is used as local oscillator 256.
Meanwhile, satellite broadcasting and communication are planned to be realized using the Ka band about in the range of receiving frequencies from 16 GHz to 24 GHz.
Referring to FIG. 13, the Ka band satellite broadcasting receiving system planned to be used for domestic COMETS is divided into an outdoor part 300 and an indoor part 310. Outdoor part 300 includes an antenna 301 and an LNB 302 connected thereto. Indoor part 310 includes an indoor receiver 304 and a terminal 308. LNB 302 amplifies a very small electric wave received at antenna 301 with reduced noise and supplies a signal at a sufficient level with reduced noise to indoor receiver 304 connected through coaxial cable 303. Indoor receiver 304 demodulates a signal input from LNB 302 using DBS tuner 305 and FM demodulator 306, and decodes data with decoder 307 for transmission to terminal 308. Terminal 308 can be for example, a so-called digital processing device such as personal computer, television set, modem and FAX.
Referring to FIG. 14, in the Ka band receiving LNB, an incoming signal at an input frequency in the range from 20.4 GHz to 21.0 GHz is received at an antenna probe 351 in a waveguide, amplified with reduced noise at an LNA 352, and then removed of images at a BPF 353. A signal passed through BPF 353 is input to an MIX 354 together with an oscillation signal at a frequency of 18.7 GHz from a local oscillator 355. The resultant signal is frequency-converted at MIX 354 into a signal in an intermediate frequency band from 1700 MHz to 2300 MHz. The signal is then amplified by an IF AMP 357 and output from an output terminal 361. A power supply 358 is a power supply to provide electric power to LNA 352, IF AMP 357 and local oscillator 356.
As a local oscillator used for receiving the Ka band can be a circuit as shown in FIG. 15. Referring to FIG. 15, the Ka band local oscillator includes an FET401 and a dielectric resonator 402. The gate terminal G of FET 401 is connected in series with a coupling line 403 and a 50.OMEGA.-terminal chip resistor 404, the other end of which is connected to ground.
The drain terminal D of FET 401 is connected to a DC power supply 414 and a capacitor 405 for grounding, the other end of which is connected to ground.
The source terminal S of FET 401 is connected to an output matching stub 406, the other end of which is connected to a coupling capacitor 407 and an inductance 408. The inductance 408 is further connected to a capacitor 409 for grounding and a chip resistor 410 for grounding, connected in parallel to the other end of inductance 408. The other ends of capacitor 409 for grounding and chip resistor 410 for grounding, are connected to ground.
The oscillation characteristics of the Ka band local oscillator such as power, frequency temperature drift, phase noise and load fluctuation are optimized depending upon the distance between dielectric resonator 402 and coupling line 403, the distance between dielectric resonator 402 and FET 401, and the width and length of output matching stub 406 provided at source terminal S.
The Ka or Ku band local oscillator is particularly difficult and costly to manufacture. This is because the circuit designing does not allow much flexibility and is difficult in optimizing the oscillation characteristic. In addition, substrate patterns cannot be readily changed as practiced according to conventional techniques in response to improvement in the oscillation characteristics derived from change in design.